1. Field of the Invention
The present invention relates to an improvement in an antenna device of an interrogator which constitutes an automatic identification system by exchanging information with an IC tag attached to an object by electromagnetic coupling. More particularly, the invention relates to a technique of maintaining a tuning frequency even when a metal body is present at the back of the antenna, thereby ensuring a communicatable distance.
2. Description of the Related Art
Identification systems which use bar codes as automatic identification means for identifying a target object have been widely used. As bar-code systems use inexpensive media for identification, are highly reliable, and have international standards, their use environments are being improved.
These bar-code systems, however, suffer from the small amount of data which is retainable on media to be identified, and they are basically designed exclusively to read data. That is, these bar-code systems cannot rewrite data.
As IC technology advances, attention is being focused on a data carrier (RF-ID) automatic identification system that has a combination of an IC tag which transmits and receives radio wave signals and an interrogator.
This automatic identification system has advantages in that the memory in an IC chip incorporated in an IC tag as a medium to be identified is rewritable and can handle a larger amount of data.
In particular, the type that activates an internal circuit of an IC tag with power supplied by electromagnetic coupling with an interrogator does not require a battery to be installed in the IC tag, which can be used almost indefinitely.
If a metal body is present at the back of the antenna device of the interrogator, however, the mutual induction of the antenna and the metal body reduces the inductance component of the antenna. This shifts the antenna resonance frequency to the high frequency side. Furthermore, the eddy current induced on the surface of the metal body increases the resistance component of the antenna, and thereby reduces the Q value of the antenna. These phenomena lower the electromotive force that is induced in the coil of the IC tag, disabling communications.
As a solution to this problem and to reduce the influence when a ferromagnetic material comes into proximity to the back of an antenna, an antenna device disclosed in Japanese Patent Application, First Publication, No. Hei 7-263936 is designed so as to retain a loop antenna in a case which constitutes a box and to have a non-magnetic material with a high dielectric constant located at the back of the loop antenna with an air layer of a predetermined thickness in between.
This structure allows the radio wave radiation pattern of the entire antenna to be nearly equal to that of a single antenna unit even if a ferromagnetic material is at the back of the antenna. This can ensure a communication distance equivalent to the one provided in the case of a single antenna unit, without reducing the electromotive force induced in the coil of the IC tag.
The aforementioned structure of the antenna device is complicated and requires a greater number of manufacturing steps, resulting in increased cost.
Furthermore, because the antenna characteristic cannot be adjusted in accordance with the environment at the antenna site, the tuning of the antenna may not always be optimal.
That is, without a ferromagnetic body, such as a metal plate, present in the vicinity of the antenna site, the particular design is not only insignificant but also reduces the Q value of the antenna.